Poor material handling practices of arsenic containing compounds and some on-site disposal has resulted in contamination of soil and groundwater at various sites. Not only is the source of the arsenic in soil due to various industrial waste processes but also from the use of lead arsenic in pesticides which was used in this country from approximately the turn of the century to the 1950's. Arsenic in herbicide manufacturing also generates much arsenic waste and also contributed to much of the contamination.
The arsenic compounds contaminating sites around the U.S. include a number of both arsenate and arsenite salts. However, these contaminated sites also contain other heavy metals, volatile and semivolatile organic compounds, and organic pesticides, notably the organochlorine pesticides.
Arsenic is exceedingly toxic to mammals. Arsenic forms poisonous compounds which, if absorbed by mammals, such as humans, causes various types of cancer, exfoliation and pigmentation of skin, herpes, polyneuritis, hematopoiesis, and degeneration of both the liver and kidneys. Acute symptoms range from irritation of the GI tract which can progress into shock and death.
Remediation of these sites is now necessary given the new Environmental Protection Agency (EPA) laws due to this extreme toxicity. The EPA has developed criteria for classifying wastes or soils as hazardous due to leaching of heavy metals, such as arsenic, in the leaching from contaminated soil. The EPA standard for arsenic leachability and non-waste water matrices is 5 mg per liter (ppm) arsenic in the leachate as measured by the Toxicity Characteristic Leaching Procedure (TCLP) leachate. Ideally, a means to solidify or chemically stabilize the arsenic and other contaminants in the contaminated soil is preferred. Preferably, the method chosen would be suitable for in-situ treatment, and would result in a volume increase of less than 10 percent in the treated soil.
Arsenic exhibits relatively complex behavior due in part to its ability to assume a range of oxidation states (-III, O, III, V) and to form organic as well as inorganic compounds. Arsenic was usually disposed predominantly in the trivalent (III) and pentravalent (V) oxidation states, as arsenite and arsenate compounds. Arsenate forms relatively insoluble compounds with calcium, iron, aluminum and copper, and is strongly adsorbed into iron and aluminum oxides and hydroxides. Arsenite compounds are generally more soluble than arsenate compounds, making arsenite more mobile and having a greater leaching ability and contamination potential. In addition, arsenite is more toxic. It is also adsorbed onto iron and aluminum oxides and hydroxides, although to a lesser degree than arsenate. This is due in part to the markedly different pH-dependence of arsenite and arsenate adsorption. The maximum adsorption for arsenate occurs at pH 4-5, whereas that for arsenite occurs at pH 9. Due to the anionic nature of arsenate and arsenite ions (above pH 9) and the negative charge developed on oxide and hydroxide surfaces under alkaline conditions, adsorption decreases dramatically at higher pH due to electrostatic repulsions.
In the past, in order to eliminate or reduce arsenic contamination, cement stabilization was used. The problem with using cement for arsenic treatment is that it has little or no effect on arsenic stabilization and does not consistently render the soil nonhazardous for arsenic leaching. Cement and cement kiln dust do not stabilize arsenic against leaching by binding it in a cement matrix as once thought. In addition, cement causes an increase in pH wherein the arsenic becomes more soluble. In addition, cement solidifies the soil causing an increase in volume and therefore an increase in cost in disposing the contaminated material. Further, cement treated contaminated soil is difficult to work with due to the change in physical properties resulting from the treatment. For arsenic contaminated soils, cement alone is not effective at doses of even 25 and 50 percent. Tests indicate that cement or cement kiln dust in combination with various salts were not effective at reducing the leachability of arsenic to the desired levels. The samples treated with cement in combination with various salts show the same degree of leachability as those samples to which only pH control additives were applied.
As previously stated, the cement treatments also lead to an increase in volume. The increase in volume for the cement-treated samples is determined by measuring the weight of soil and final volume of the cement treated samples.
The 25 percent cement treatment resulted in a 54 percent increase in volume for the laboratory sample, while the 50 percent treatment resulted in an 82 percent volume increase.
One stabilization approach that can be used is the addition of ferric iron salts as demonstrated by McGaham U.S. Pat. No. 5,252,003 ('033 patent) in which ferric salt in combination with magnesium oxide is used to stabilize arsenate contaminated wastes or soils. However, one problem not addressed by the '033 patent is that the ferric iron may be reduced to ferrous iron in land disposal environments. Ferrous iron is not effective at stabilizing arsenic. The ferrous arsenate salts are much more soluble than the ferric salts. Arsenic may be released into ground water from the treated waste if such a reduction occurs.
Organic binders were also used to stabilize arsenic-contaminated material. Organic binders are also not preferred due to the fact that they also increase volume similar to that of cement and, therefore, increase the cost of eliminating the contaminated material.